Wiring in thermostat without circuit board

Looking for help wiring in my new thermostat to an old hvac unit with no circuit board.

I’ve got 4 wires at my current thermostat:
red, green, white, and blue. I found these same wires feeding to the unit, as pictured. How do I connect the adapter?

I had a similar setup with no control board. I was missing the C wire. I used a multimeter to find the 24v between two wires. For mine, the red wire was one side of the 24v transformer and I found a black wire that was on the other side of the 24v transformer. This became my C wire. You will need to look up the model number of your furnace and you should be able to find a wiring diagram.

Based on your picture, I would guess the brown wire would be the other side of the transformer. Check voltage between red and brown wires.

If you get 24v, then you would connect the red, green, white, blue wires that are bundled together to the adapter, then plug the other end of the adapter back into those original wire nuts, then the brown wire should be your C wire.

I don’t have a C wire at the thermostat. Isn’t that why I need the adapter?

I ran into the same situation with my 51 year old furnace. I was lucky though and actually had an unused 5th wire going to the thermostat. I found the other wire from the 24 VAC transformer in the furnace control box and connected it to the big red wire in this photo running up to the thermostat. So I was able to eliminate the C-wire adapter.

Yes, you need the adapter if you don’t have an extra wire going back to the thermostat to use as a C wire.

I also had some extra unused wires going back to my thermostat, so I used one for the C wire.

I want to use the adapter. My question is how do I wire it?

You will label the red, green, white, blue wires just like you labeled them behind the thermostat and remember what wire they were connected to in the wire nut. Then connect them to the included adapter. Then the pigtail end of the adapter will connect where those wires were in the wire nuts. Then if you get 24v to that brown wire, you will add the C wire on the pigtail to the wire nut with the brown wire. The adapter will basically be between the thermostat wires and the wire nuts.

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Assuming the blue wire in your thermostat bundle is actually a Y wire, C is likely to be the brown wire that feeds the red wire for the outdoor unit.

This is assuming you have an air conditioner, and that’s not a float switch for a boiler.

Also, the red wire leading to the outdoor unit looks to have a spot where the insulation has rubbed off or split, you should wrap it with electrical tape to prevent a short circuit.

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Yes. I can confirm that the blue wire would be “my” yellow wire.

And, yes…we have an A/C, not a boiler unit.

Thanks for the tip on taping the red wire. I’ll do that!

I’ve got time to work on this today. I’ll report back with how it goes!

“I get by with a little help from my friends”

Got it done! Everything works just fine. Thanks for the help!

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Glad you got it working!

Hi Speadie,
Can you help with mine? - I only have 4 wires. Where does the c-wire from the adapter connect?

Actually here’s a better pic. Turns out I have an unused white wire

Is this inside or outside your house? The location of the copper tubing leads me to think this is the outside unit? Or do you have a unit that is ducted through the wall? Where is the blower fan for your system located?

My mistake if I was supposed to start a new thread!

Same issue, Im used to automotive and an old hvac just nskes ne timid. I know its steaight forward, but cant make the call! Where am i supposed to hijack a common. The 2 whote coming out from one side of the box: 1 feeds blue to outside, the other feeds white to thermostat. ((IM GUESSING THE BLUE IS THE “C” NUT ILL TAP INTO?Tester wont read anything except hot on both red and briwn coming out of the unit obviously, multimeter read 26 at thermostat between a ground and red. And , yep, confused mysrlf again haha. Please help, and thanks in advance? Also trying to figure out the photo upload, corneal transplsnts make it very difficult to figure out these little symbols
Blue and yellow go to outside unit, also there are 4 unused on both ends of thermostate wiring

At thr current thermostat red is R-V. White-W-H, Yellow-Y-C, And Green-G-F

  • i guess i was just assuming the adapter would just plug the 4 existing in the appropriate spots. And the other (5) side woykd run to thermostat, be easy enough to mount durectly next to it ; or in the wall. I understand now why thats not the case, but i also did not expect to see a “C” terminsl on BOTH sides of the “adapter,” seems silly, or im not " clicking" something. And considering i ran 26 lights, myltiple horns, effects. And audio video thru breakers and some step downs in my truck, not to negate the fact my father INVENTED the twin break transfer switch! ( ive got a bit of know how/ just enough to get me in trouble so im asking for help before i break something and have to call the colonel.
    Patent#4,791,255 dec 13,88 if i get fsct checked lol

You should be able to get your C from the same wire nut that the blue wire from outside goes to. Just use the blue wire to the thermostat if it’s available and you shouldn’t need the adapter.


Had already slipped my mind that all 4 "extra " are available top and bottom… geez louise lol THANK YOU. You also have the initials of a dear friend of mine Peter Christian lol. So Thanks bud, i was nearly having an anxuety attack with thought of calling my father about this…BIG Speech about not going to cornell like him always starts it off

Yep, just connect the blue wire from the bottom thermostat cable to the 2 blue and 1 white wire nut at the top, and then find the blue wire in the wall at your thermostat and connect that to C.

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